The preferred site of Cr or Mo alloying element in the DO₃-Fe₃Al intermetallic compounds, and the effect of Cr or Mo on the mechanical properties and electronic structure of Fe₃Al are investigated by first-principle based on density functional theory. The research demonstrated that Cr or Mo preferred substitutes the Al atom in Fe₃Al, and Cr or Mo increases the bulk moduli, rigidity moduli and Young"s modulus of Fe₃Al and improves the ductility of Fe₃Al. According to the analysis of the electronic structure and the electronic density, the effect on improving the mechanical properties of Fe₃Al can be mainly attributed to the s, p, d-state orbital electron of the element Cr or Mo dropped is involved in hybridization of Fe₃Al, Cr or Mo has increased the number of the bonding peaks of Fe₃Al and overlapping electron number among atoms of Fe₃Al, in addition, Cr or Mo decrease the directionality of electron atmosphere between Fe-Fe of Fe₃Al.